After upgrading to the most recent version of GP, with premium plugin, I can no longer customize the child blogs on ScienceBlog.com. This is the error from the logs from a sample:
[Fri Dec 08 17:34:48.386601 2017] [:error] [pid 3413] [client 127.0.0.1:64343] PHP Catchable fatal error: Object of class WP_Customize_Image_Control could not be converted to string in /nas/content/live/scienceblogcom/wp-content/object-cache.php on line 565, referer: https://gaiacantelli.scienceblog.com/wp-admin/themes.php?page=generate-options
Thanks Tom. the only cache is WP Engine’s default caching behavior, which CAN be turned off, but which I’d rather not disable for performance reasons. I guess I will tackle option 1, deactivating all plugins, but this is really a brute force approach for a site getting steady traffic. :-/